Spatial Light Modulators (SLMs) are transducers that modulate an incident beam of light in a spatial pattern that corresponds to an optical or electrical input. A type of SLM is the SLM based on Micro Electro-Mechanical Systems (MEMS). A typical MEMS-based SLM consists of an array of micro-mirrors mounted on movable elements. Each individual micro-mirror can be independently deflected by an electrostatic force. Reflection of a beam of incident light impinging a micro-mirror can then be controlled, for example by deflecting the micro-mirror through changing the electrostatic force applied to the micro-mirror. MEMS-based SLMs have experienced significant developments and been innovatively implemented in many applications, one of which is the use in digital display systems. In a display application, each micro-mirror is associated with a pixel of a displayed image. To produce a bright pixel, the state of the micro-mirror associated with the pixel is set in such a way that the reflected light from the micro-mirror is directed onto a target for viewing. And to produce a dark pixel, the status of the micro-mirror is tuned such that the reflected light from the micro-mirror is directed away from the display device. In order to display a black-and-white image, the micro-mirror array is illuminated by a beam of light. By coordinating the reflective status of the micro-mirrors based on the brightness of the pixels of the desired image, the collective effect of all reflected lights from individual micro-mirrors is the generation of the desired image. Gray-scaled and colored-images can also be displayed using the micro-mirror array with the pulse-width-modulation and sequential-color-display techniques, which will not be discussed in detail herein.
Currently, varieties of MEMS-based SLMs have been developed. Regardless of the differences, the function of the MEMS-based SLMs for display is based on the reflection of light from individual micro-mirrors. Therefore, the quality of the displayed image strongly depends on the reflections of the micro-mirrors.
There are many things that define the quality of a displayed image. Contrast ratio is a major determinant of perceived image quality. Contrast ratio is the ratio of luminance between the brightest white that can be produced and the darkest black that can be produced. If a displayed image has high contrast ratio, a viewer will judge it to be sharper than a displayed image with lower contrast ratio, even if the lower contrast image has substantially more measurable resolution. Contrast ratio of a displayed image from a MEMS-based SLM can be seriously degraded by light scattered, for example, from the edges of the micro-mirrors and the structures below the micro-mirrors. This scattered light typically travels through the projection lens of the display device and is directed on to the target, even when the micro-mirrors are set for displaying a dark pixel.
Therefore, methods for use in MEMS-based SLMs are needed to improve the display quality.